Ironman 70.3 Training: What Is Zone 2 / Endurance?
SUMMARY:
Zone 2 training sits at 73–80% of max heart rate, 56–75% of FTP and 87–94% of CSS swim speed (RPE 3–4). This is the endurance zone that drives Ironman 70.3 performance. It feels easy to moderate, fully aerobic and repeatable over long durations. Zone 2 builds deep cardiovascular fitness, improves fat metabolism and prepares your body to meet the long-course demands of swim, bike and run on race day.
What Is Zone 2 Training?
When training for an Ironman 70.3, success isn’t just about pushing harder, it’s about training smarter. One of the most powerful tools in your endurance arsenal is Zone 2 training, also known as the “aerobic base zone.” Zone 2 refers to a low-intensity aerobic training zone where your body primarily uses fat as a fuel source. Sessions in this zone are done at a comfortable, steady pace, think “conversational effort.”
Zone 2 Training Metrics
Heart Rate: 73–80% of your max heart rate (HRmax)
Power (Cycling): 56–75% of your FTP (Functional Threshold Power)
Perceived Effort: Easy to moderate; sustainable for hours
CSS swim speed: 87–94%
FLJUGA Training Calculators. Set your zones and train smart.
Unlike higher-intensity efforts, Zone 2 training helps you build endurance without burning out, making it essential for Ironman athletes.
Why Is Zone 2 Training Important for Ironman 70.3?
Ironman 70.3 races are long. Between the swim, bike and run, you’ll be active for 4–8 hours. That means your aerobic system must carry the load and that’s where Zone 2 shines.
1. Builds Deep Aerobic Endurance
Zone 2 boosts mitochondrial density, oxygen delivery and capillary development, all crucial for long-distance racing. With a strong aerobic base, your body can handle longer efforts with less fatigue.
2. Improves Fat Utilisation
Burning fat efficiently helps conserve glycogen, your body’s limited carbohydrate store. This delays fatigue and energy crashes during your race. Zone 2 trains your body to become a fat-burning machine.
3. Enhances Recovery and Lowers Injury Risk
Because it’s low stress, Zone 2 training reduces strain on muscles and joints. That means you can increase training volume safely while recovering faster and minimising injury risk.
4. Supports Pacing and Execution
Zone 2 builds your engine. Come race day, it helps you maintain a steady pace and prevents early burnout, especially in the latter half of the run.
How to Use Zone 2 Training in Your Ironman 70.3 Plan
1. Prioritise Zone 2 in Base Training
During the first phase of your Ironman 70.3 prep, aim to spend 70–80% of your weekly training in Zone 2. This lays the groundwork for more intense sessions later on.
2. Focus on Long Workouts
Bike: Weekly long rides (2–4 hours) in Zone 2 are key for building endurance.
Run: Zone 2 runs of 60–90 minutes develop aerobic capacity and mental resilience.
Swim: Long continuous swims (e.g., 3×800m) at a moderate pace help build comfort and efficiency in the water.
3. Use Zone 2 for Recovery Days
Zone 2 isn’t just for building endurance, it’s also a powerful active recovery tool. Easy Zone 1/2 sessions after hard workouts promote circulation and muscle repair.
4. Blend with Race-Specific Workouts
In the final 8–10 weeks before your race, add tempo and race pace intervals into your Zone 2 sessions to simulate Ironman efforts.
Example Brick Session:
Bike: 3-hour Zone 2 ride with 3×20 min at race pace
Run: 75-minute Zone 2 run with the last 20 min at race pace
Example Zone 2 Workouts for Ironman 70.3 Athletes
Swim – Aerobic Endurance Set
3×800m at Zone 2 pace (30 sec rest between sets)
Focus on smooth, efficient strokes and consistent pacing
Bike – Long Ride for Fat Adaptation
3–4 hour ride in Zone 2
Optional: Add 3×10 min at race pace in the final hour
Run – Aerobic Base Builder
60–90 minutes steady in Zone 2
Maintain controlled breathing and good form throughout
How to Know You’re in Zone 2
Not using heart rate or power yet? Here are three simple ways to check:
Talk Test: You can hold a conversation without gasping
Nose Breathing: You can breathe comfortably through your nose for most of the session
Effort Check: It feels “easy,” but you’re still getting the work done
FLJUGA Training Calculators — set your zones and train smart.
Triathlon-Specific Benefits of Zone 2 Training
Zone 2 training doesn’t just build endurance, it directly supports your performance across all three Ironman 70.3 disciplines:
Improves pacing control: Helps you stay smooth and steady across the swim, bike and run without surging too early
Boosts energy efficiency: Enhances fat metabolism, reducing your need for constant fueling mid-race
Supports brick training: Builds the aerobic durability needed to handle long bike-run sessions back to back
Reduces fatigue accumulation: Allows you to train consistently without digging too deep into your recovery reserves
Zone 2 is how you build a strong, reliable engine that holds up over 70.3 miles, even when the course gets tough.
How Zone 2 Fits Into Your Ironman 70.3 Week
Zone 2 should make up the bulk of your Ironman 70.3 training, especially during your base phase and early build.
Typical Weekly Zone 2 Structure:
Swim: 2× per week, 1,500–2,000m at an easy aerobic pace
Bike: 2–3× per week, including 2–3 hour long ride at 56–75% FTP
Run: 2× per week, including 60–90 min steady Zone 2 runs
Brick Sessions: Long Zone 2 bike followed by a short aerobic run off the bike
Aim to complete 70–80% of your total weekly training volume in Zone 2 to build lasting endurance without overloading your system.
Common Mistakes in Zone 2 Training
Zone 2 is incredibly effective, but only if executed with precision.
Watch out for these common mistakes:
Training too hard: Drifting into Zone 3 by mistake, especially on runs or hills
Expecting instant results: Endurance gains from Zone 2 take time and repetition
Neglecting fueling: Long aerobic sessions still need proper nutrition and hydration
Skipping recovery: Even with Zone 2 volume, your body still needs full rest weeks
Stick to the plan consistently, remain patient throughout the process and allow the aerobic base to do its quiet, steady work beneath the surface.
FAQ: Zone 2 Training for Ironman 70.3
How often should I train in Zone 2 each week?
In base-building phases, 4–6 sessions per week in Zone 2 is ideal. Keep at least 70% of your total volume easy.
Can I lose weight with Zone 2 training?
Yes! Since Zone 2 relies on fat as a primary fuel source, it can support healthy fat loss when combined with proper nutrition.
How long should Zone 2 sessions be?
Start with 45–60 minutes and build to 2–4 hour rides or 90-minute runs as your endurance improves.
Should I use a heart rate monitor or power meter?
Both are great tools. For cycling, power is ideal. For running, a heart rate monitor is simple and effective.
FURTHER READING: BUILD YOUR IRONMAN 70.3 BASE
Ironman 70.3 Training: What Is Zone 3 / Tempo?
Ironman 70.3 Training: What Is Zone 4 / Threshold?
Ironman 70.3 Training: What Is Zone 5 / VO2 Max?
Ironman 70.3 Swim Training: 10 Essential Sessions
Ironman 70.3 Bike Training: 10 Essential Sessions
Ironman 70.3 Run Training: 10 Essential Sessions
Ironman 70.3 Training: Recovery Week
Ironman 70.3: Beginner’s Guide
Final Thoughts
Zone 2 training serves as the essential foundation for achieving strong Ironman 70.3 performance. By consistently training at these lower intensities, you'll effectively build the endurance engine necessary to power through long, grueling workouts and excel on race day itself. It’s important not to underestimate the significant benefits of going slow in order to get fast. By committing fully to your Zone 2 workouts over time, you’ll set yourself up to reap substantial rewards when it truly counts on race day.
Ready to build endurance the smart way and let Zone 2 unlock your race-day potential?
Always consult with a medical professional or certified coach before beginning any new training program. The information provided is for educational purposes only and is not a substitute for personalized advice.
